1. Technical Field
The invention relates to a method and apparatus for measuring optical properties of passive optical devices in dependence upon both wavelength and polarization state using a so-called matrix method.
The invention is especially, but not exclusively, applicable to the measurement of polarization dependent center wavelength (PDCW) and polarization dependent bandwidth (PDBW).
2. Background Art
Polarization dependent properties of an optical device, such as polarization dependent loss (PDL), are measured by passing light through the device and measuring the output power while varying the state of polarization through a wide range of states of polarization. For accuracy, a large number of measurements are taken so as to cover most of the Poincarxc3xa9 sphere. The number of measurements may be reduced by using the so-called Mueller matrix method as disclosed in, for example, International Electrotechnic Commission standard IEC 61300-3-12 1997-02 and in U.S. Pat. No. 5,371,597 (Favin et al.), specifically by computing PDL by taking measurements with only four distinct states of polarization, one of which is elliptical, and computing only the four elements of the first row of the Mueller matrix. These four elements allow the output power to be computed for any input state of polarization. This reduces the number of measurements significantly.
It is desirable to be able to measure optical properties of an optical component over a range of wavelengths. More particularly, it is desirable to be able to measure polarization dependent center wavelength (PDCW) and polarization dependent bandwidth (PDBW) because, in DWDM systems, standards define wavelength ranges within which devices must operate. If PDBW is too large, there is an increased risk of crosstalk. If PDCW is too great, the wavelength could stray towards the end of the range. In either case, signal power would be compromised.
An object of the present invention is to provide a method and apparatus for effecting such optical property measurements efficiently.
According to one aspect of the present invention, a method of measuring a polarization and wavelength dependent property of an optical device comprises the steps of.
(i) passing through the device an optical signal and varying the wavelength of the optical signal over a wide range of wavelengths;
(ii) selecting four unique states of polarization of the optical signal, at least one of the unique states of polarization being elliptical;
(iii) measuring the optical signal leaving the device and determining the insertion loss of the device for each of the four states of polarization and at each wavelength;
(iv) using the four insertion loss measurements for each of the four different states of polarization at each wavelength to compute the four elements of the first line of the Mueller matrix for each wavelength; and
(v) using the Mueller matrix elements, computing a set of insertion loss variations with wavelength for the device for each of a multiplicity of input states of polarization substantially covering the Poincarxc3xa9 sphere, and computing said polarization and wavelength dependent property from the computed sets of insertion loss variations.
Variation of the wavelength may be achieved by varying the wavelength of the optical signal before application to the device, or by using a broadband optical source and performing the measurements at each of a plurality of wavelengths in the range.
The step of computing said polarization and wavelength dependent property may include the steps of selecting two sets of insertion loss variations exhibiting the maximum bandwidth and minimum bandwidth, respectively, at a prescribed insertion loss and computing therefrom a polarization dependent bandwidth (PDBW).
Additionally or alternatively, the step of computing said polarization and wavelength dependent property may include the steps of selecting the two insertion loss variations exhibiting the maximum and minimum center wavelength, respectively, and computing therefrom the polarization dependent center wavelength (PDCW).
The three unique states of polarization other than the elliptical state conveniently may comprise three substantially linear states of polarization.
The states of polarization may be selected by means of a polarization state adjustor before application to the device under test. Alternatively, they may be selected by means of a polarization state analyzer after the optical signal has passed through the device under test.
According to a second aspect of the invention, apparatus for measuring a polarization dependent property of an optical device comprises:
a tunable optical source for supplying an input optical signal to an optical device under test (DUT) and varying its wavelength over a wide range of wavelengths, or a broadband source for supplying the input optical signal to the DUT and a tunable measuring system for detecting the output optical signal and performing measurements over a prescribed range of wavelengths;
polarization state selection means for selecting four unique states of polarization of the optical signal, at least one of which is elliptical;
means for measuring the optical signal leaving the DUT to determine insertion loss of the DUT for each of said four polarization states at each of said wavelengths;
means for computing from the four measurements for each wavelength the four elements of the first line of the Mueller matrix, and, using the Mueller matrix elements, computing a set of insertion loss variations with wavelength for the device for each of a multiplicity of input states of polarization substantially covering the Poincarxc3xa9 sphere, and computing said polarization and wavelength dependent property from said sets of insertion loss variations.
The means for computing said polarization and wavelength dependent property may be arranged to select the two insertion loss variations exhibiting the maximum bandwidth and minimum bandwidth, respectively, at a prescribed insertion loss, for example, relative to the peak transmission, and compute therefrom a polarization dependent bandwidth (PDBW).
Additionally or alternatively, the computing means may be arranged to select the two insertion loss variations exhibiting the maximum and minimum center wavelength, respectively, and compute therefrom the polarization dependent center wavelength (PDCW).
The three unique states of polarization other than the elliptical state of polarization conveniently may comprise three substantially linear states of polarization.
The polarization state selection means may comprise a polarization state adjustment device for adjusting the state of polarization of the optical signal before application to a port to which, in use, the device under test will be connected. Alternatively, it may comprise a polarization state analyzer connected to a port which, in use, will be connected to receive the optical signal from the device under test.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of a preferred embodiment of the invention, taken in conjunction with the accompanying drawings.